The invention pertains to a method in which a plurality of objects of a similar type are simultaneously polished on a polishing installation provided with two polishing plates that are actively operated in parallel. The invention also pertains to a corresponding polishing installation. It is already known that a very wide range of types of objects are mechanically polished to obtain a high-quality, planar surface. Suitable polishing installations (CMP, chemical mechanical polishing installations) are commercially available for these applications. These installations are in use, for example, in various sectors of the semiconductor fabrication industry.
By way of example, semiconductor wafers, in particular silicon wafers, have to be polished as gently as possible after they have been patterned with the corresponding circuit parts. In the semiconductor industry, it is preferred to use polishing installations with two or more polishing plates, on which the silicon wafers are processed simultaneously, to polish these silicon wafers. In this process, first of all the individual silicon wafers are positioned with the surface to be polished on the polishing plates by means of suitable carriers. The polishing process then begins, with all the silicon wafers being polished simultaneously until the desired result is achieved. Then, unpolished silicon wafers have to be fitted to all the polishing plates again, so that a new polishing operation can be started.
In practice, it has been found that FA polishing cloths (fixed abrasive polishing cloths) are particularly suitable for this polishing operation. An abrasive which is used to process the surface of the silicon wafers is embedded in these polishing cloths. However, this method using FA polishing cloths has the drawback that the state of the polishing cloth and therefore the result of polishing are determined to a considerable extent by the structure of the surface of the silicon wafer which is to be polished. For example, if partially polished, patterned silicon wafers were to be transferred to a different polishing plate, the non-uniform topology or different degree of wear of the FA polishing cloths could lead to an unsatisfactory result of polishing. For this reason, the operation of polishing a silicon wafer is always carried out entirely on the same polishing plate. The additional mounting time required for loading and unloading is grudgingly accepted.
As an alternative to polishing plates with adhesively bonded polishing cloths, it is also possible to use plate structures in which the polishing cloth is in the form of a strip and is tensioned between two rollers, of which one roller holds the unused polishing cloth and of which the used cloth is wound onto the other roller. In this case, the polishing-cloth strip is typically wound a certain distance from one roller onto the other between two polishing processes, so that each silicon wafer is in contact with the same proportion of used and unused polishing cloth during the polishing process.
It is accordingly an object of the invention to provide a method of simultaneously polishing a plurality of similar-type objects on a polishing installation with a plurality of polishing plates, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which is optimized so that the objects are polished with the plurality of polishing plates operated actively in parallel with one another.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for simultaneously polishing a plurality of objects of a similar type in a polishing installation with at least two polishing plates disposed diagonally opposite one another with respect to a central axis, and at least four carriers for holding the objects to be polished. A rotary cross connects the four carriers to one another, the rotary cross is rotatably mounted about the central axis and disposed substantially coaxially with respect to the at least two polishing plates, such that in each position of the polishing installation, in each case one carrier comes to lie opposite a polishing plate and at least one further carrier is located in a loading and unloading position for the object. The following steps are included in the novel method:
dividing each polishing operation into at least first and second polishing sections;
rotating the two carriers opposite the two polishing plates, together with the objects to be polished, through 180xc2x0 between the first and second polishing sections; and
during an ongoing polishing operation, exchanging the object at the carrier located in the loading and unloading position of the polishing installation in each polishing section.
In accordance with an added feature of the invention, each polishing section is defined to correspond to approximately half a polishing time.
In a preferred embodiment, after a polishing operation has ended, one of the two carriers located opposite the two polishing plates is rotated, together with the polished objects, into the loading and unloading position for the object.
With the above and other objects in view there is also provided, in accordance with the invention, a polishing installation, comprising:
at least two polishing plates disposed diagonally opposite one another with respect to a central axis;
at least four carriers for holding objects to be polished, and a rotary cross connecting the four carriers to one another;
the rotary cross being rotatably mounted about the central axis, and being disposed coaxially with respect to the at least two polishing plates;
wherein, in each position of the polishing installation, in each case one of the carriers lies opposite one polishing plate, and at least one further the carrier is located in a loading and unloading position for the object;
wherein at least first and second polishing-installation positions are defined for each polishing operation, and the two carriers lying opposite the two polishing plates, together with the objects to be polished, are rotated through 180xc2x0 between the first and second polishing-installation positions; and
wherein it is possible, during an ongoing polishing operation, to exchange an object on the carrier located in the loading and unloading position in each of the polishing-installation positions.
The invention is suitable in particular for all polishing processes in which the state of the polishing cloth and therefore the result of polishing are influenced by the structure of the polished silicon wafers.
The method according to the invention for simultaneously polishing a plurality of objects of a similar type on a polishing installation having two polishing plates which are actively operated in parallel, and the corresponding polishing installation according to the invention, have the advantage that no or only a little additional machine running time is required for loading and unloading of the objects, so that in practice the polishing operation can be carried out in the shortest possible time. In this context, the fact that the ongoing polishing operation is only interrupted briefly for rotation of the rotary cross, so that on the one hand a xe2x80x9cnewxe2x80x9d carrier is moved into position for changing the wafer and, secondly, it is possible to compensate for different polishing properties of the polishing plates, is considered particularly advantageous. The result of polishing is improved and the process becomes more stable.
Since, in the proposed polishing method according to the invention, there is always one carrier at a free position, during the ongoing polishing process it is advantageously possible for the object to be loaded and unloaded, so that the polishing operation can be shortened considerably.
A further advantage is that despite the interruptions the polishing process can be continued with an FA polishing cloth, since all the exchanged objects and the associated FA polishing cloths are in the same state. It is therefore possible to successfully reduce differences in the results of polishing.
To achieve the maximum possible throughput of polished objects, it appears favorable for the duration of a polishing phase to be determined as a function of the time which is required for the loading and unloading of the free carrier.
It is also favorable for the number of interruptions in the ongoing polishing process to be determined as a function of the number of actively operated polishing plates. For example, if only a few polishing plates are required, the number of interruptions required to the ongoing polishing operation is also reduced. This likewise leads to a quicker throughput.
Furthermore, it is advantageous for this method to be applied even if not all the available polishing plates of a polishing installation are operated with the same polishing method. The best polishing results are achieved if, for example in the case of a polishing installation with three polishing plates, only two polishing plates are operated actively, these two polishing plates being arranged diagonally opposite one another. In this case, only one interruption is required in order to achieve an optimum throughput. The inactive polishing plate could then remain unused. This arrangement has the advantageous result that, when changing the two processed silicon wafers after an interruption, the two associated FA polishing cloths also have the same abrasion properties.
It is also considered favorable for an unused polishing plate to be available for a further application in a polishing process. This results in the advantage that this polishing plate can be used, for example, for a conventional polishing operation, for example for preliminary cleaning or for finishing polishing, without the polishing process on the other polishing plates being impeded.
A favorable alternative solution is also regarded as lying in an arrangement in which a plurality of free carriers are fitted with unpolished objects. This could be carried out using a suitable mechanical device. This further accelerates the loading and unloading operation.
A preferred application of the polishing method according to the invention is considered to lie in the arrangement in which silicon wafers are used as the objects. These silicon wafers can be processed at low cost using the FA polishing process in particular after they have been patterned.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for simultaneously polishing a plurality of objects of a similar type, in particular silicon wafers, on a polishing installation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.